1.6 TDI CAYC - Rough idle when engine is half-warm

My 1.6 TDI has fluctuating revs for couple minutes after I start the engine. Rev needle goes up and down and I can also hear engine sound changing. This only happens with half-warm engine, not cold or fully warmed up. While driving, there isn't any issues and rough idle goes away after engine warms up.

So what could it be? This has been happening for 2 years but now it has gotten slightly worse.

No fault codes or vacuum leaks. All injectors have good values and this also happens when MAF isn't plugged and EGR is closed then too. I also noticed that using electric devices makes rev fluctuating much worse. For example, I have 3 x 55W Hella's halogen extra lights and switching on high beams usually triggers this issue. My car is equipped with 180A alternator and 1kw PTC which only works in cold weather.